Hi,
I have a script here
interleave :: [Integer] -> [Strings] -> [String]
interleave (x:xs) (y:ys)
= words [a | a <- (unwords [(show x), (filter (/= 1) y), "+"])]
Firstly, "Strings" is not a standard type. I suspect
you mean "String" so the type signature is
interleave :: [Integer] -> [String] -> [String]
Now consider your sub-expression
filter (/= 1) y
y is a string (as it is the first element of a list of strings)
filter will use it as a list of characters though because
String == [Char] in haskell and filter expects a list.
1 is a number; the haskell type is Num a => a
which means: 1 is of type a provided that it is an instance
of class Num (that is: provided that it is a number)
All usual number types are instance of class Num:
Int, Integer, Double, Float
Filter will compare 1 against each element of the character list,
using /=.
Let's say: y = "abc".
In list notation: y = ['a', 'b', 'c']
now filter will attempt three comparisons
1 /= 'a'
1 /= 'b'
1 /= 'c'
However, the compiler detects that characters ('a', 'b' and 'c' here)
are not numbers, so this comparison is invalid.
Conclusion:
If you try to compare an expression which yields a number with
some expression which doesn't yield a number, hugs will tell you
that the second expression "Char is not an instance of class Num" or
"instance of Num Char required"
which you should read: "Char is not a number and it needs to be
in order to be compared with a number".
